Basics of RTK GPS and GNSS for non-surveyors

• How GNSS positioning works
• What is RTK GPS and how does it work
• Reach RS3 for RTK
• FAQs 

Global Navigation Satellite System or GNSS influences every industry: navigation to farming, construction, and archaeology. In this article, we’ll discover how GNSS positioning and the related Real-Time Kinematic (RTK GPS) technique work.

How GNSS positioning works

Global Navigation Satellite System (GNSS) is the standard generic term for navigation satellite systems. GNSS receivers are capable of using various navigation satellite systems, while GPS receivers can only use the navigation satellite system called the Global Positioning System. Due to the widespread use of the term GPS to refer to both types of receivers, the term GPS is used as the generic term in this help documentation. 

The whole GNSS system, and RTK GPS as a part of it, is based on measuring how long it takes for a signal to travel from a satellite to the receiver.

You can determine your position on the planet, knowing the precise orbits of the space vehicles (the ephemeris) and travel times from at least four satellites—to first form the area and then determine the exact location by finding the point where all the spheres cross. To get a closer look into positioning, check out our Positioning With GNSS video.

When you determine your position, you may wonder how accurate it is. Most of the GNSS or GPS receivers, like the one you can find in your smartphone or on most robotic platforms, determine your position with 2-4 meters accuracy. It’s enough for you to find a shop you were looking for on Google Maps. However, the estimation is way too rough for, say, land surveying. So what’s the source of such an error, and how to eliminate it?

To address this issue, high precision GPS technology such as an RTK GPS system comes into play. These systems provide the accuracy needed for applications that require precise measurements, like surveying equipment. They significantly reduce errors by correcting the signal distortions and providing much tighter control over positioning, essential in professional surveying and construction projects.

What is RTK GPS and how does it work

Real-Time Kinematic (RTK) is the technique that eliminates errors as much as possible to give you accurate results and enhanced positional data down to centimeter resolution.

When the signal from satellites travels towards the receiver, it goes through 20,200 km of ionosphere and atmosphere down to Earth. The ionospheric effect significantly slows the signal and also can disturb it on the way. In addition, many factors, such as clouds, or obstacles, can affect the travel time and increase the position error.

To deal with these issues, RTK comes to the rescue. For RTK, you need two GNSS receivers: one is static and called a “base station,” the other is moving and is called a “rover.” While both receivers observe the same satellites simultaneously, the static base station is placed at a point with known coordinates (a benchmark or a point measured beforehand). Taking into account the known coordinates and receiving satellite signals, the base transmits corrections to the moving rover. This way, the rover can get sub-centimeter accurate positioning. The idea is simple, but the devil’s in the math here. Find out more about the Real-Time Kinematic in our guide How RTK works.

You don’t necessarily need two units for RTK all the time. You can also perform RTK with only one rover. In this case, you’ll need local services that share base corrections over the Internet and are called NTRIP. Acting as a base station, the NTRIP service transmits the corrections to your rover. For more information about working with NTRIP, check out our article What Is NTRIP. 

If you need to survey in an area without Internet coverage required for NTRIP corrections, there’s another solution for you. PPK or Post-Processed Kinematic technique works pretty much like RTK, but without a real-time connection between the base and the rover. Instead, both units record raw data during the survey, and then these logs are processed together in PPK software. To learn more about PPK, watch our tutorial How PPK Works.

Reach RS3 for RTK

Reach RS3 is an excellent fit for both RTK GPS and PPK scenarios. With tilt compensation, Reach RS3 enables accurate results at large tilt angles. This versatility makes Reach RS3 a valuable piece of surveying equipment for professionals across various industries.The receiver comes with a handy Emlid Flow app and supports data import in industry-standard formats. You can easily export the data from Reach RS3 to popular GIS or CAD services for further processing.

Reach RS3 is available in the Emlid online store and ship worldwide. You can also buy Reach RS3 from official Emlid dealers—find your local dealer here to get started.